There's a few docs online from one of the oil field "auditors" (the ones that value reserves and help measure risk, advise on investing and so are familiar with the science) and it looks to me from those reports that there's a good chance that everyone knows why the well blew out. The BOPs failing is a separate subject. A BOP are like airbags in a car. They help mitigate the damage, and the BOPs didn't. What it looks like is that the cement job failed, and the design of the pipe in the hole didn't allow for a casing hanger. Start with this document:
Look at Schematic #3. You'll see the 7" x 9 7/8" (tapered) casing is run to surface, through the 9 7/8" lnr (not run to surface)
There is a space and the possibility that the blowout happened from poor cement across the oil/gas formation and then between the 7" and 9 7/8" liner. It would have a free run all the way up to the base of the BOP.
This also implies the 7" x 9 7/8" casing is still viable and still has cement plugs in place. If all true, then it also means that this well would have blown out with heavy mud in the casing. For the heavy mud to get down in a large 9 7/8" space with the oil flowing is one thing, as it's being engineered for. For that same heavy mud to get into a much smaller space , the space between the 9 7/8" pipe and the 16" casing (again, look at the red line/arrows in the diagram) with the oil and gas "jetting out" is going to be much tougher. What may happen is the heavy mud goes in, and gets rejected out, and _then_ the call goes out to put in the junk, stopping up the flow partially, and then trying more heavy mud. They've got plenty of mud, so they say, so they'll try this to see what happens.